1. Technical Field
This invention relates to olefinic polymers. More particularly, it relates to a process for the preparation of olefinic copolymers using metallocene catalysts.
2. Background of the Art
Over the last several years it has been shown by a number of researchers that use of metallocene catalysts offer improvements in polymer properties when compared with production of similar polymers using Ziegler-Natta catalysts. For example, metallocene catalyzed polyolefins typically have lower xylene solubles than Ziegler-Natta catalyzed polymers. At least some of the improvements are attributable to the increased control of stereochemistry, molecular mass and comonomer incorporation obtainable with the metallocenes. This control enables tailoring of the final polymer and may offer improvements in areas as diverse as optical performance, strength performance, aging and the like.
For many applications, including films, fibers and injection molded articles, copolymers including a proportion of ethylene have been found to be both useful and economical. Because of this, a variety of such copolymers have been identified as exhibiting improved properties when prepared with certain metallocene catalysts. The incorporation of ethylene into these materials has been problematic, however. This is, in part, because ethylene incorporation may frequently occur in a percentage that is less than the percentage of ethylene in the feedstock. However, higher levels of ethylene incorporation offer the benefit of reducing the melt temperature of the final copolymer, which leads some manufacturers to employ proportionately larger amounts of ethylene in the feedstock. Unfortunately, this approach is not always trouble free and may cause undesirable effects, such as, for example, excessive venting or circulation problems, which may make it difficult to control the polymerization process. Use of relatively large amounts of ethylene in the feedstock may also detrimentally affect the quality of the copolymer being produced. For example, in some cases it may increase the production of xylene solubles. When this occurs, the copolymer's performance in its ultimate application, such as a film, may be unacceptable.
Thus, what is needed is a process for preparing copolymers with the ability to produce low melting metallocene random copolymers without the use of excessively high ethylene concentration.